1. Field of the Invention
The present invention relates to universal serial bus systems, and, in particular, to powering universal serial bus systems.
2. Description of the Related Art
Computers and other devices commonly use Universal Serial Bus (USB) communication ports for serial data transfers. A USB system generally consists of one or more host devices and one or more peripheral devices connected in a tiered-star topology. Host devices include, for example, computers, smartphones, and video game consoles. Peripheral devices include, for example, flash drives, printers, keyboards, smartphones, and various other types of transceivers. USB standards are defined in a series of specifications. USB 2.0 Specifications, Revision 2.0, hereinafter “USB 2.0”, describes a common serial data transfer interface used in a wide variety of applications. USB 3.0 Specifications, Revision 1.0, hereinafter “USB 3.0”, describes a more recent interface that builds from USB 2.0 and requires backward capability to interface with USB 2.0.
USB interfaces commonly supply electric power to peripheral devices, and, therefore, many devices connected to a USB interface generally do not need a power source of their own. A USB 2.0 interface provides 5 volts (V) to connected devices through a pin in its connecting cable. A USB 2.0 connecting cable typically is comprised of a power line, a ground line, and two data differential lines. A USB 3.0 connecting cable accommodates the USB 2.0 lines, and has two additional data differential lines. Systems that require a bus-powered application might use 5V from the USB 2.0 cable's pin that is connected to the power line. Other USB peripheral devices, commonly referred to as self-powered devices, include their own on-board power supply and, therefore, do not make use of the 5V bus for operating power. USB 2.0 also requires a 3.3V signal swing for full-speed signaling at the physical layer (PHY) of an upstream (e.g., peripheral) device. Therefore, an integrated USB 2.0 system that is compatible with both USB 2.0 and USB 3.0 standards has access to a 3.3V supply.
Existing approaches to an integrated USB 2.0 PHY solution often come at a high cost, such as by requiring an external 3.3V source supplied to the PHY layer circuits. Other approaches are generally unable to efficiently integrate USB 2.0 PHY with USB 3.0 PHY because the USB 2.0 PHY layer employs circuitry having 2.5V Input/Output (I/O) devices, which generally increases power consumption. USB 2.0 PHY also requires additional circuitry to detect bus voltage in integrated solutions.